Don't have the experience of settling down later in life as such (however, one that does sprig to mind is George Clooney 😂), however I can see where your brother is coming from.
I'm late 30s and frankly I haven't been with anyone where being with that person doesn't feel like 'settling for less'. It's not that I am looking for something crazy, it's more that I am hypercritical of people and if they make my life more draining rathern than easier and better, I keep them at arm's length. I don't really fall in love.
I did fall in love in my very early 20s and he seemed like the perfect match, for many many years I thought he was 'the one' while being with other people, and almost chasing that feeling of what being with him felt like.
It's also not that I date 'below par' men. A few examples of relationships or potential relationships I've pulled out of:
- MP - super interesting person but thought he had mental health and substance abuse issues
- CEO of a household name company - likewise, substance abuse and extreme levels of stress/unhappiness
- Premier League footballer - extremely attention-seeking, lifestyle revolved around parties, women, alcohol
- Model - very good-looking but also very insecure, lacking direction in life
- Tech guy - unconcerned with external presentation or social grace, extremely matter-of-fact, antisocial
- 'Normal' guy - again, antisocial, thinking of anything like a day out or trip to another city as a big deal
- Guy from work - life revolved around drinking and football, social life heavily revolved around seeing his family
- Another guy from work - focused on his own career progression and having an easy life, often complaining, no real thought about wider social issues or problems of the world
As you can see, I am able to find something wrong with absolutely everyone, whereas I am sure many women would find these men quite dateable, especially the higher income ones.
Or perhaps I attract difficult personalities and they are single for a reason, who knows.
What would I like? I'd like someone established in what they are doing, perhaps an artist, scientist or writer, or another profession that revolves more around their purpose and less around making money, that has a calm, thoughtful nature and curiosity about the world, broad range of topics etc...
Would I find that? Maybe, but not easy. My first thoughts upon meeting somebody is usually outlining what is wrong with them - unless it's attraction at first sight, which is extremely rare.
I don't mind if I stay single-ish for life with a string of short relationships, a bit like your brother, as long as I am true to myself and happier outside a relationship than in one. Being in a relationship is not a goal in itself for me.
At this stage of life I feel like I can relate strongly to the 'eternal bachelor' lifestyle. I do need connection and intimacy, but I am unwilling to commit to something/ someone that negatively disrupts a lifestyle that works well for me and isn't aligned with my identity and choices.
Apologies if that's not what you were looking for on here, but perhaps this helps you understand your brother's mind better.